Photodiode structures

ABSTRACT

Photodiode structures and methods of manufacture are disclosed. The method includes forming a waveguide structure in a dielectric layer. The method further includes forming a Ge material in proximity to the waveguide structure in a back end of the line (BEOL) metal layer. The method further includes crystallizing the Ge material into a crystalline Ge structure by a low temperature annealing process with a metal layer in contact with the Ge material.

FIELD OF THE INVENTION

The invention relates to semiconductor structures and, moreparticularly, to photodiode structures and methods of manufacture.

BACKGROUND

A photodiode is a semiconductor device that converts light into current.In use, the current is generated when photons are absorbed in thephotodiode. Photodiodes may contain optical filters, built-in lenses,and may have large or small surface areas depending on the application.

The material used to make a photodiode is critical to defining itsproperties. This is mainly because only photons with sufficient energyto excite electrons across the material's bandgap will producesignificant photocurrents. Some materials used in photodiodes includemetal wiring, silicon and germanium.

Crystalline germanium can be used as an optical detector; however ifgermanium is deposited in an amorphous form and it is not in contactwith crystalline silicon it will require a long anneal at 450° C. to550° C. in order to crystallize. This high temperature will result inmany grain boundaries. Also, such high temperatures can destroy metallines in the photodiode during the fabrication processes.

SUMMARY

In an aspect of the invention, a method comprises forming a waveguidestructure in a dielectric layer. The method further comprises forming aGe material in proximity to the waveguide structure in a back end of theline (BEOL) metal layer. The method further comprises crystallizing theGe material into a crystalline Ge structure in the dielectric materialby a low temperature annealing process with a metal layer in contactwith the Ge material.

In an aspect of the invention, a method comprises forming a waveguidestructure in a dielectric material. The method further comprises forminga Ge material in proximity to the waveguide structure. The methodfurther comprises forming at least one via in the dielectric material toexpose a surface of the Ge material. The method further comprisesforming a metal seed layer on sidewalls of the at least one via and incontact with the surface of the Ge material. The method furthercomprises crystallizing the Ge material by a low temperature annealingprocess with the metal seed layer in contact with the Ge materialthrough a nucleation process.

In an aspect of the invention, a structure comprises: a waveguidestructure and metal wiring layers in a dielectric material; acrystalline Ge structure formed in proximity to the waveguide structurein the dielectric material; and at least one metal filled via inelectrical contact with the Ge material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIGS. 1-5 show respective structures and fabrication processes accordingto an aspect of the present invention;

FIG. 6 shows a structure and respective fabrication processes accordingto additional aspects of the present invention; and

FIG. 7 shows a structure and respective fabrication processes accordingto additional aspects of the present invention.

DETAILED DESCRIPTION

The invention relates to semiconductor structures and, moreparticularly, to photodiode structures and methods of manufacture. Inmore specific embodiments, the photodiode structures are metal inducedlateral crystallized germanium (Ge) photodiodes. Advantageously, themetal induced lateral crystallized germanium (Ge) photodiodes will haveimproved electrical and optical performance.

In embodiments, the crystallization of the photodiode can be, forexample, provided by annealing a metal contact on germanium material ofthe photodiode using a low temperature anneal, e.g., 350° C. to 420° C.The low temperature anneal can be part of a standard metal contactprocess and structure, e.g., back end of the line (BEOL) processes. Themetal contact can be a Ni contact to Ge to lower the annealcrystallization temperature. That is, Ni (or another metal as describedherein) will act as a nucleation site for Ge, which will have the effectof lowering the overall thermal budget (a function of temperature andtime) to crystallize the Ge and result in larger grains.

It should be noted that if the Ge is not in contact with the metal toact as this nucleation site (catalyst) such as Ni, it would take ahotter temperature upwards of 450° C. to 550° C. to crystallize the Ge,which would also create smaller crystal grains. Accordingly, byimplementing the processes of the present invention, the Ge will forminto a recrystallized structure with large single crystal regions (e.g.,lateral crystallized germanium structure) in a dielectric material, andnot a smaller polycrystalline structure which has degraded electricaland optical performance.

In embodiments, the region of Ge material can have several crystallizedregions each large, e.g., larger than a few microns in length. This isobtained by using the seed window, e.g., Ni contact to Ge (without theseed window polycrystalline Ge will form, with 10×poorer dark current.)The large grain size will reduce the number of crystallized regions,thereby reducing the total amount of grain boundaries. This, in turn,will improve optical performance by reducing light scattering andimproving dark current.

In further embodiments, other metals are contemplated by the presentinvention for the seed window including, for example, all forms ofgermanides, e.g., Co, Pd, etc., or all forms of eutectics, e.g., Au, Ag,Al, etc. The structures of the present invention can be used in BEOLmetal stack, as well as in a package or on a board, for example.

More specifically, the optical detector, e.g., waveguide structure andGe material (photo detector structure), is formed in the dielectricmaterial of the wiring layers of a printed circuit board, package or thewiring levels of a semiconductor chip. In any of these embodiments, theGe material is provided in the dielectric layer of any particular wiringlevel at a BEOL; instead of being in contact with a single crystallinesilicon material in a front end of the line (FEOL) process, as inconventional structures. In each of these applications of the presentinvention, the photo detector (e.g., germanium) will be deposited in anamorphous deposition process in a BEOL wiring layer, followed by a lowtemperature anneal (nucleation process) to be crystallized for bestopto/electrical properties.

The photodiode structures of the present invention can be manufacturedin a number of ways using a number of different tools. In general,though, the methodologies and tools are used to form structures withdimensions in the micrometer and nanometer scale. The methodologies,i.e., technologies, employed to manufacture the level translator of thepresent invention have been adopted from integrated circuit (IC)technology. For example, the structures of the present invention arebuilt on wafers and are realized in films of material patterned byphotolithographic processes on the top of a wafer. In particular, thefabrication of the level translator of the present invention uses basicbuilding blocks, including: (i) deposition of thin films of material ona substrate, (ii) applying a patterned mask on top of the films byphotolithographic imaging, and (iii) etching the films selectively tothe mask.

FIGS. 1-5 show respective structures and fabrication processes accordingto aspects of the present invention. More specifically, in FIG. 1, thestructure 10 includes a substrate 12, e.g., interlevel dielectric layerssuch as an oxide. The structure 10 further includes wiring and contactlayers generally depicted at reference numeral 14. The wiring andcontact layers 14 can be fabricated using conventional CMOS processes,e.g., deposition, lithography and etching (reactive ion etching (RIE))during back end of line (BEOL) processes. For example, the wiring andcontact layers 14 can be formed using conventional subtractive oradditive processes.

Illustratively and by way of brief explanation, at appropriate wiringlevels of the dielectric layer 12, a metal can be deposited (e.g., usingchemical vapor deposition (CVD), plasma enhanced CVD (PECVD) or otherappropriate deposition methods) on a surface of the dielectric layer 12and a photoresist can be formed thereon. The photoresist can be exposedto energy (e.g., light) in order to form a pattern. Through conventionaletching processes, e.g., reactive ion etching (RIE) with appropriatechemistries, a corresponding pattern (vias) is formed in the wiring andcontact layers 14. This can be used to form contacts and wiring, etc.above a silicon layer and FEOL structures, depending on the pattern,design and level of the structure. The photoresist is then removed usingconventional processes, e.g., oxygen ashing processes. An oxide or otherinsulator material is then deposited about the wiring and contact layers14 to form additional interlevel dielectric layers 12.

In an additive process, a dielectric layer will be patterned and etchedto form an area for both wires and vias, and a metal, e.g., copper,tungsten, etc., deposited within the pattern to form the wiring andcontact layers 14. Any residual material is removed from the surface ofthe dielectric material using, e.g., a chemical mechanical process(CMP).

Still referring to FIG. 1, a waveguide structure 16 is formed on asurface of the dielectric layer 12. In embodiments, the waveguidestructure 16 can be a silicon material or nitride material, depending onthe particular application of the device and other design criteria. Inembodiments, the waveguide structure 16 can be formed by conventionaldeposition, lithography and etching (reactive ion etching (RIE))process, as already described herein. The waveguide structure 16 can beplanarized using conventional CMP processes. In the case of thewaveguide being silicon, a barrier layer 18, e.g., Si₃N₄, can bedeposited and patterned on the waveguide structure 16 using conventionalCMOS processes.

In FIG. 2, a layer of Ge material (photo detector) 20 is deposited overthe waveguide structure 16, in an amorphous state. In embodiments, theGe material 20 can also be formed below the waveguide structure or inproximity thereto as already shown in FIG. 2. In preferred embodiments,the layer of Ge material 20 is amorphous Ge formed in the dielectriclayer 12, as compared to a conventional photodiode where the Ge materialis formed in direct contact with the single crystalline substrate inorder to crystallize the Ge material. In this case, because the Ge is incontact with crystalline silicon, the anneal temperature torecrystallize Ge is low and there are not yet metal lines that cannottolerate the recrystallization anneal. The layer of Ge material 20 canbe formed using conventional deposition, e.g., CVD, and patterningprocesses. The layer of Ge material 20 is patterned (e.g., usinglithography and etching processes) to form the structure shown in FIG.2. In embodiments, the layer of Ge material 20 can be offset from acenter with the waveguide structure 16; although, other positions andlocations as described herein are also contemplated by the presentinvention. Additional dielectric layers 12 can then be deposited overthe layer of Ge material 20 and planarized using a CMP process.

In FIG. 3, a dual damascene process is performed to form vias andtrenches 22. In embodiments, two single damascene processes can also beperformed in order to form the vias and trenches 22. The trenches exposethe layer of Ge material 20. In preferred embodiments, the vias andtrenches 22 are positioned to not interfere with light entering thewaveguide structure 16. In addition, in embodiments, the vias andtrenches 22 are positioned such that a halfway point between them is notover the waveguide structure 16. Accordingly, as noted already, thelayer of Ge material 20 can be offset with respect to the waveguidestructure 16, e.g., not centered on the waveguide structure 16.

As representatively shown in FIG. 4, a metal seed layer 21, e.g., nickel(Ni), is deposited within the vias and trenches 22. More specifically,the seed layer 21 is formed on sidewalls of the at least one via andtrench structures and in contact with a surface of the Ge material 20.It should be understood that FIG. 4 representatively shows the seedlayer 21 only in a single via and trench for ease of explanation;however, in this embodiment, the seed layer 21 is deposited in both (orall) of the vias and trenches 22. In embodiments, the metal seed layer21 will provide nucleation of the layer of Ge material 20 and lower itstemperature for lateral crystallization processes. The metal seed layer21 is not limited to Ni, and can be, for example, all forms ofgermanides, e.g., Co, Pd, etc., or all forms of eutectics, e.g., Au, Ag,Al, etc.

Still referring to FIG. 4, the structure then undergoes a lowtemperature annealing process at about 350° C. to 420° C. in order toresult in a heat reaction between the Ge and the seed layer, therebybeginning a nucleation process to form a crystalline Ge structure. Inembodiments, the nucleation begins at the metal layer, e.g., nickel,forming a capping layer 24 as a result of the nucleation process. Asshould be understood by those of skill in the art, nucleation can occurin two stages. In the first nucleation stage, a small nucleus containingthe newly forming crystal is created at the initial site, e.g., at theinterface of the Ge amorphous material and the metal seed layer. Aftercrystal nucleation, the second stage of growth rapidly ensues, where thecrystal growth spreads outwards from the nucleating site such that theGe material will be in a crystalline form as nucleation continues awayfrom the initial site. In this way, the Ge material will be crystallizedrelative to the waveguide structure 16.

In this example, the nucleation site, e.g., capping layer 24, can be acompound of NiGe, using Ni as the seed layer. Also, as described above,the Ni (or other metal) will act as a nucleation site for Ge,effectively of lowering the overall thermal budget (a function oftemperature and time) to crystallize the Ge and result in larger grains.For example, the nucleation process will crystallize the Ge layer 20 ata lower temperature, e.g., about 350° C. to 420° C., without the need tobe in contact with a single crystalline silicon in a FEOL structure.This low temperature process thus allows the crystalline Ge material toform in the BEOL metal layer, which improves its electrical and opticalcharacteristics while not damaging any of the metal lines. In addition,this process provides flexibility in forming the crystalline Ge materialin any metal layer, compared to being restrained by forming the photodetector in contact with a silicon at a higher temperature process inthe FEOL processes.

In embodiments, the crystallized Ge layer 20 can undergo a volume changeduring the heating process; however, such volume change, e.g.,expansion, can be accommodated by the additional space provided by thevias and trenches 22. In this way, the integrity of the crystallized Gelayer 20 will remain intact, e.g., the crystallized Ge layer 20 will notcrack the encapsulating dielectric.

In embodiments, a boundary layer 28 can also form in the crystallized Gelayer 20; however, this boundary layer is not provided over thewaveguide structure 16 due to the positioning of, e.g., the crystallizedGe layer 20 and/or vias and trenches 22. Also, any remaining unreactedmetal in the vias and trenches 22 can be cleaned using an etch process,for example as shown representatively in the rightmost via and trench ofFIG. 4. Although FIG. 4 shows the metal seed layer 21 in the leftmostvia and trench, this is provided merely for explanation of thedeposition of the seed layer and one of skill in the art wouldunderstand that this metal seed layer 21 is also removed duringsubsequent processes, as described herein. Also, in optionalembodiments, the capping layer 24 can be removed. In this optionalembodiment, metal which subsequently fills the via and trenches will bein direct physical and electrical contact with the crystallized Gelayer.

As shown in FIG. 5, the vias and trenches are filled with metal material26, in contact with the crystallized Ge layer 20. By way of example, thevias and trenches can be lined with metals such as: Ta, TaN, or Ti, thenfilled with copper or other metal. The copper can then be planarizedusing a CMP process, to result in the structure shown in FIG. 5.Although not part of the present invention, additional processes cancontinue including packaging etc.

FIG. 6 shows a structure 10′ and respective fabrication processesaccording to additional aspects of the present invention. In particular,in this embodiment, the metal seed layer, e.g., nickel (Ni), isdeposited in only one of the vias and trenches 22′ to form a cappinglayer 24. In embodiments, the metal seed layer will provide nucleationof the layer of Ge material 20 as described above, in order tocrystallize the layer of Ge material 20 at low temperatures. Again, themetal seed layer is not limited to Ni, and can be, for example, allforms of germanides, e.g., Co, Pd, etc., or all forms of eutectics,e.g., Au, Ag, Al, etc.

Still referring to FIG. 6, to crystallize the Ge material (e.g., form arecrystallized structure with large single crystal regions on the orderof a few microns in length) and to form the capping layer 24, thestructure undergoes an annealing process at about 350° C. to 420° C.This process crystallizes the Ge layer 20 at a lower temperature, e.g.,about 350° C. to 420° C., which improves its electrical and opticalcharacteristics while not damaging any of the metal lines. Also, sincethe seed layer is provided in only a single via and trench structure,this process may eliminate the boundary layer, hence, the waveguidestructure 16 can be positioned at other locations with respect to thecrystallized Ge layer 20 and/or the via and trench 22′. Also, anyremaining unreacted metal in the via and trench 22′ can be cleaned usingan etch process, for example, with the option of removing the cappinglayer 24. The vias and trenches can be filled with metal material 26, incontact with the crystallized Ge layer 20, as already described herein.Although not part of the present invention, additional process cancontinue including packaging, etc.

In FIG. 7, the structure 10″ includes a single via and trench structure22″, which includes the recrystallized Ge material with large singlecrystal regions (e.g., lateral crystallized germanium structure) andcapping layer 24 formed by heat reaction as described with regard toFIGS. 1-6, e.g., annealing process at about 350° C. to 420° C. in orderto result in a heat reaction between the Ge and the seed layer. That is,a metal seed layer, e.g., nickel (Ni), is deposited in the via andtrench 22″, formed remotely from the waveguide structure 16. Inembodiments, the metal seed layer will provide nucleation of the layerof Ge material 20 as described above, which assists in the crystallizingof the Ge material at the low temperature. Again, the metal seed layeris not limited to Ni, and can be, for example, all forms of germanides,e.g., Co, Pd, etc., or all forms of eutectics, e.g., Au, Ag, Al, etc. Ametal interconnect 30, e.g., tungsten, can also connect an underlyingmetal layer 14 to the crystallized Ge layer 20. The metal interconnect30 can be formed using conventional CMOS processes as described herein.

In embodiments, the structure 10″ of FIG. 7 may eliminate the boundarylayer, hence, the waveguide structure 16 can be positioned at otherlocations with respect to the crystallized Ge layer 20 (e.g.,crystalline Ge structure) and/or via and trench 22″. Also, any remainingunreacted metal in the via and trench 22″ can be cleaned using a wetetch process, for example, with the option of removing the capping layer24. The vias and trenches can be filled with metal material 26, incontact with the crystallized Ge layer 20, as already described herein.Although not part of the present invention, additional process cancontinue including packaging, etc.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed:
 1. A method, comprising: forming a waveguide structurein a dielectric layer; forming a Ge material in proximity to thewaveguide structure in a back end of the line (BEOL) metal layer; andcrystallizing the Ge material into a crystalline Ge structure by a lowtemperature annealing process with a metal layer in contact with the Gematerial.
 2. The method of claim 1, wherein the low temperatureannealing process is at about 350° C. to 420° C.
 3. The method claim 2,wherein the metal layer is a metal seed layer formed in direct contactwith the Ge material within a via formed to expose a surface of the Gematerial, the low temperature annealing is performed after deposition ofthe metal seed layer is formed in direct contact with the Ge material,within the via of the dielectric layer.
 4. The method of claim 1,wherein the metal layer is a metal seed layer of Ni in contact with theGe material.
 5. The method of claim 1, wherein the metal layer is agermanide or a eutectic.
 6. The method of claim 1, wherein any unreactedmetal layer is removed after the low temperature annealing process. 7.The method of claim 1, wherein the metal layer is a metal seed layerdeposited in a via and on a surface of the Ge material.
 8. The method ofclaim 1, wherein the metal layer is a metal seed layer deposited in twovias in a dielectric material composing BEOL wiring layers, offset froma center of the Ge material.
 9. The method of claim 1, wherein the metallayer is a metal seed layer deposited in one of at least two vias. 10.The method of claim 1, wherein the Ge material is amorphous Ge prior tothe low temperature annealing.
 11. The method of claim 1, wherein thecrystallizing of the Ge material comprises: forming at least one via ina dielectric material to expose the Ge material; forming a metal seedlayer in the at least one via; annealing the metal seed layer at atemperature of about 350° C. to 420° C. to form a capping layer on theGe material and to laterally crystallize the Ge material; and removingany unreacted metal seed layer.
 12. The method claim 11, furthercomprising filling the via with metal, in contact with the Ge material.13. The method of claim 12, further comprising removing the cappinglayer such that the metal in the via is in direct contact with themetal.
 14. The method of claim 1, wherein the forming of the Ge materialin proximity to the waveguide structure in a back end of the line (BEOL)metal layer comprises: depositing and patterning a barrier layerdirectly on the waveguide structure; and depositing and patterning thelayer of Ge material on the barrier layer.
 15. The method of claim 14,wherein the barrier layer is nitride.
 16. The method of claim 1, whereinthe crystallizing the Ge material into the crystalline Ge structurecomprises: depositing the metal layer as a metal seed layer directly onthe Ge material which has been exposed by etching a trench and via inthe dielectric layer; and laterally crystallizing the Ge material by ananneal process between 350° C. and 420° C.
 17. The method of claim 16,wherein the metal seed layer is formed on sidewalls of the trench andvia that exposes a surface of the Ge material prior to the deposition ofthe metal seed layer.
 18. The method of claim 17, further comprisingforming a boundary layer in the crystallized Ge material, which ispositioned to a side of the waveguide structure.